The present invention relates to a coextruded, biaxially stretched and laminated heat-shrinkable tubular film, comprising:
a gas-barrier layer of a copolymer of vinylidene chloride having a thickness, which is not more than 30% of the total thickness of the coextruded, biaxially stretched and laminated heat-shrinkable tubular film, of not less than 6 .mu.m, outer layers of a polyolefin, at least one intermediate layer of a polyamide or a thermoplastic polyester, both of which show a crystal melting point of not more than 240.degree. C., said intermediate layer having a thickness of 5 to 40% of the total thickness of the coextruded, biaxially stretched and laminated heat-shrinkable tubular film, and adhesive layers disposed between all or certain polymer layers, and
the total thickness of the coextruded, biaxially stretched and laminated heat-shrinkable tubular film being in the range of 25 to 110 micrometers,
the coextruded, biaxially stretched and laminated heat-shrinkable tubular film being produced by laminating and co-extruding the copolymer of vinylidene chloride, the polyolefin, polyamide or the thermoplastic polyester and the adhesive material, cooling, heating and stretching the resultant laminated tubular film.
More in detail, the present invention relates to a coextruded, biaxially stretched and laminated heat-shrinkable tubular film, comprising:
(1) a gas-barrier layer of a copolymer of vinylidene chloride having a thickness, which is not more than 30% of the total thickness of the coextruded, biaxially stretched and laminated heat-shrinkable tubular film, of not less than 6 .mu.m,
(2) outer layers prepared from at least one polymer selected from the group consisting of:
(i) a copolymer of ethylene and vinyl acetate having a crystal melting point ranging from 80.degree. to 103.degree. C.,
(ii) a linear low-density polyethylene having a crystal melting point ranging from 110.degree. to 130.degree. C., and
(iii) a mixture of more than 0% and not more than 40% by weight of a lineat low-density polyethylene having a crystal melting point ranging from 110.degree. to 130.degree. C., not less than 60% and less than 100% by weight of a copolymer of ethylene and vinyl acetate having a crystal melting point ranging from 80.degree. to 103.degree. C., and
(iv) a mixture of more than 40% by weight and less than 100% by weight of a linear low-density poly-ethylene of a density in the range of 0.915 to 0.930 and more than 0% by weight and less than 60% by weight of a copolymer of ethylene and vinyl acetate of a crystal melting point in the range of 80.degree. to 103.degree. C.,
the outer layers having a thickness of 40 to 92% of the total thickness of the coextruded, biaxially stretched and laminated heat-shrinkable tubular film,
(3) at least one intermediate layer of a polyamide or a thermoplastic polyester, both of which show a crystal melting point of not more than 240.degree. C., said intermediate layer having a thickness of 5 to 40% of the total thickness of the coextruded, biaxially stretched and laminated heat-shrinkable tubular film, and
(4) adhesive layers disposed between all or certain polymer layers, each adhesive layer having a thickness of 0.5 to 5.0 .mu.m, and
the total thickness of the coextruded, biaxially stretched and laminated heat-shrinkable tubular film being in the range of 25 to 110 micrometers,
said coextruded, biaxially stretched and laminated heat-shrinkable tubular film being produced by laminating and co-extruding the copolymer of vinylidene chloride, the polyolefin, polyamide or the thermoplastic polyester and the adhesive material, cooling, heating and stretching the resultant laminated tubular film.
For packaging the foodstuffs non-uniform and irregular shape, a method of heat-shrink packaging is generally applied, and since the thus packaged foodstuff should be preserved safely for a long time period, it is necessary that the packaging used in such a packaging is impermeable to gases, particularly to gaseous oxygen, in other words, the packaging material must have a gas-barrier property.
As the heat-shrinkable gas-barrier film for packaging the foodstuffs, a single film of a copolymer of vinylidene chloride (hereinafter referred to as VDC) has been utilized. However, because of the poor mechanical properties of the single film of VDC at low temperatures, a composite laminate film comprising a gas-barrier layer of VDC and an outer layer of polyolefin has been developed.
For example, U.S. Pat. No. 4,309,466 discloses a composite, flexible packaging film consisting essentially of an outer web of a balanced biaxially-oriented polyamide;
a middle web of metallized thermoplastic material,
an inner web of heat-sealable polyolefin; and
at least one of said outer and middle webs having a vinylidene chloride polymer coating thereon.
U.S. Pat. No. 4,133,924 discloses a heat shrinkable laminate film which comprises a heat shrinkable synthetic resin film and a low shrink film which forms creases or crepes when said laminate is heat shrunk, and which is laminated on the heat shrinkable film with an adhesive composition having low cohesive force and high melt flowability and an adhesive strength lower than the shrinking force of the heat shrinkable film.
French Patent No. 2516017 discloses a heat shrinkable laminated film comprising an inner layer of a gas impermeable resin comprising a vinylidene chloride copolymer or an ethylene/vinyl alcohol copolymer, an outer layer (A) made of a resin which is a copolymer (I) of ethylene and .alpha.-olefin having a density of 0.900-0.950 and a crystalline melting point of 110.degree. to 130.degree. C. or a mixture of more than 20 weight % of the copolymer (I) and less than 80 weight % of a copolymer (II) of ethylene and vinyl acetate having a crystalline melting point of 80.degree. to 103.degree. C., an outer layers (B) comprising a resin made from a crosslinked material formed from one of the copolymer (I) and/or the copolymer (II) by exposing the polymer to ionising radiation.
Also, European Patent No. 0051480 discloses laminate films for heat-shrink packaging foodstuffs comprising a center layer and two outermost layers with a respective adhesive layer between the center layer and each outermost layer, and being capable of exhibiting a rate of heat shrinkage at 90.degree. C. of more than 15%, the center layer consisting of a copolymer (I) of vinylidene chloride and both the outermost layers consisting of from 20 to 70% by weight of a copolymer (II) of ethylene and an alphaolefin, the copolymer (II) having a crystalline melting point of 110.degree. to 125.degree. C., and from 80 to 30% by weight of a copolymer (III) of ethylene and vinyl acetate, the copolymer (III) having a crystalline melting point of 85.degree. to 103.degree. C., the content of copolymer (II) in the total amount of copolymers in both the outermost layers being less than 65% by weight.
U.S. Pat. No. 4,188,443 discloses a heat shrinkable, multi-layer, flexible, thermoplastic packaging film comprising:
(a) at least three layers of polymeric material;
(b) at least one of said layers being an inner layer and comprising a polymer selected from the group consisting of: (i) homopolymers, copolymers, and terpolymers of ethylene and (ii) blends of ethylene homopolymers or copolymers with butylene homopolymers or copolymers, said inner layer comprising 50% to 90% of the total thickness of said laminate;
(c) another of said layers comprising a polymer selected from the group consisting of polyesters and copolyesters, said layer being stretch oriented;
(d) said multi-layer film having shrink tension in the range from 100 to about 400 p.s.i.; and,
(e) said film having a total thickness in the range between 0.1 mil and 2.0 mil.
Further, U.S. Pat. No. 4,289,830 discloses a multi-layer packaging film structure comprising an outer structural polymer layer of nylon or a polyolefin with a modified polyolefin adhesive layer, a gas-barrier layer consisting essentially of ethylenevinyl alcohol polymer to which has been added an orthosubstituted phenol having a melting point below about 185.degree. C. and a boiling point above about 125.degree. C., an adhesive layer of a modified polyolefin, an inner structural layer of a polyolefin, and an inside heat sealable layer.
Also Japanese Patent Application Laid-Open (KOKAI) No. 58-128821 (1983) discloses a laminate film comprising a gas-barrier layer of a vinylidene chloride copolymer (hereinafter referred to as VDC) and outer layers of a copolymer of ethylene and vinyl acetate (hereinafter referred to as EVA), and the disclosed film has solved the problem of the poor mechanical properties at low temperatures. However, because of the lack of rigidity in such a film, such a laminate film cannot exhibit a satisfactory operational processability in packaging foodstuffs.
As a result of the present inventors' studies, the problem of the lack of rigidity has been solved by incorporating a layer of a polyamide or a thermoplastic polyester between the gas-barrier layer and the outer layer of polyolefin of the laminate film.